Dear Miss Jirawan,
Thank you for confirming that the O2 sensor will in fact be located above the tank and not inside it.
1. and 2. I'm not sure from your drawing if the tanks are topped? Or are the contents open to the air? If the gases within the tank are exposed to the open air, then yes there may be significant corrosion to the sensor. The O2 sensor will not directly read Styrene or Methyl Methacrylate. And if the tank is closed off at the top, there should be only minimal exposure to the gases. But again if the tank top is open, we recommend that you pass on the application, as the sensor will probably corrode significantly.
3. The Oxygen sensor, as well as the SM95 / 4-20IQ electronics, are warranted for one year. The sensor has an expected lifetime of 1-3 years.